Supplementary Material for: Sexual Dimorphism in Facial Morphology and Skin Color in the Horn of Africa

Introduction: Previous research has shown substantial variation in sexual dimorphism of facial structure and skin color across human populations. This study investigates sexual dimorphism in both facial shape and skin color in understudied populations from the Horn of Africa – Somalis and Ethiopians – focusing on the relationship between facial morphology and pigmentation traits. Methods: Standardized frontal and profile photographs were collected from participants and analyzed using geometric morphometrics. Sexual shape dimorphism (SShD) was calculated by projecting each individual's facial shape onto a vector connecting average male and female shapes in Procrustes-aligned morphospace; higher values indicate more masculine morphology. Sexual color dimorphism (SCoD) was computed similarly, by projecting individual skin color values (from forehead and cheeks) onto a vector defined by average male–female difference in the CIE Lab* color space. Trajectory analysis and Bayesian hierarchical modelling were performed to examine associations between SShD and SCoD. Results: Significant sexual dimorphism was detected in both facial shape and skin color across both populations. Male faces tended to be more robust and darker, while female faces were more gracile and lighter-skinned. However, despite group-level sex differences, individual-level associations between facial structure and skin pigmentation were weak or absent. Somali participants exhibited greater dimorphism in skin color compared to Ethiopians, while facial shape dimorphism remained consistent between groups. Conclusion: Both target populations revealed significant sexual dimorphism in both structural and pigmentation facial traits. However, in contrast to previous findings reported in certain West African populations, we found no statistically reliable association between these two traits at the individual level. This decoupling may indicate distinct evolutionary or ecological pressures shaping morphological and pigmentation traits. Our findings suggest that no universal trade-off or consistent relationship exists between morphological and pigmentation components of sexual dimorphism across human populations.